4-Tert-butyl-n-[6-(2-hydroxyethoxy)-5-(2-methoxyphenoxy)-2-(2-pyrimidinyl)-pyrimidine-4-yl]-benzenesulfonamide, monohydrate also known as Bosentan monohydrate is represented by structural formula (I).

Bosentan is first disclosed in, the U.S. Pat. No. 5,292,740 and used in the treatment of pulmonary arterial hypertension (PAH) to improve exercise capacity and symptoms in patients with grade III functional status.
The U.S. Pat. No. 5,292,740 (hereinafter referred as US'740) discloses a method for the preparation of Bosentan sodium (formula III) involving the reaction of chloropyrimidin benzenesulfonamide (formula II) with sodium metal in ethylene glycol. Bosentan sodium (formula III) is employed for the preparation of Bosentan Formula I)

The process according to the patent US'740 leads to the formation of undesired ethylene glycol bisulfonamide along with Bosentan which show negative impact on yield of Bosentan. This ethylene glycol bisulfonamide is also known as dimer impurity. Hence Bosentan. prepared according to the patent US'740 requires repeated purification to get desired purity.

The patent US'740 discloses crystalline Bosentan sodium salt having characteristic peaks of XPRD shown in FIG. 1 and melting point of 195 to 198° C.
The PCT publication WO 2011/058524 discloses form C and form D of Bosentan sodium salt characterized by XPRD values and the process for preparing the same thereof. The PCT publication WO2012056468 discloses the crystalline Bosentan Lithium salt and the process for preparing the same thereof.